I don't know of any bags that are bigger and fit in the ski locker.. does not seem to be any space when mine is full.
Couple of options... tailgates description above, or an integrated bow sac under the front seats. You could add a 4th valve to your ballast system and an additional drain pump and make that work. There are a couple of posts on this topic on here.. search gravity iv or 4 .
I am content with 1200 lbs in mine. I don't enjoy driving my boat over loaded.. but I am not pulling any inverts or many tricks at all for that matter..

I am just a 38 yr old that enjoys riding.. gettin a little air and gettin out in the sun/water.. No style, steeze or special stuff in my riding! HA HA

If you run 1500 in the rear, you'll need at least 1000 in the front to balance it out.. I ran 1500 in rear, center locker full and 300 on top of center locker once last year and still porpoised a little.. of course, you could load the bow with boat bun and that might help, but you'll need at least 4 of those..